Project Summary Epilepsy is a chronic condition that can develop after various neurological insults, including traumatic brain injury, stroke, and infection. However, despite years of study, the mechanism for the development of epilepsy from an initial neurological insult has remained elusive. In the hippocampal dentate gyrus, seizures drive pathologic retrograde sprouting of granule cell mossy fiber axons. Although sprouting may contribute to epileptogenesis, the physiologic consequences of mossy fiber sprouting are unknown. Additionally, the dentate gyrus is a locus for adult neurogenesis and adult-born granule cells might directly give rise to sprouted mossy fibers, though this is widely debated and unresolved. In our hands, preliminary data has indicated that adult- born granule cells do give rise to sprouted mossy fibers, which form large boutons in the inner molecular layer of the dentate gyrus. Moreover, whole-cell patch clamp recordings from mature granule cells indicate that these adult-born neurons functionally contribute to recurrent excitatory circuitry. The goal of this project is to better understand mechanisms that contribute to hippocampal hyperexcitability and provide a targetable mechanism for future treatments to prevent epileptogenesis. Our overall hypothesis is that adult-born granule cells provide the bulk of mossy fiber sprouting and that these aberrant synapses create recurrent microcircuits that drive runaway excitation within the hippocampus. This hypothesis will be tested using the pilocarpine model of epilepsy and transgenic labeling of age-defined cohorts of granule cells in mice. Mossy fiber sprouting will be quantified by immunofluorescent staining of granule cells, and whole-cell electrophysiology will be used to detect recurrent circuits and interrogate the sprouted synapse using optogenetic stimulation of granule cell cohorts. The specific aims of this proposal independently examine the contribution of adult-born granule cells to mossy fiber sprouting and the physiological properties of a sprouted synapse. Furthermore, this proposal investigates whether silencing of sprouted mossy fibers can attenuate the hippocampal hyperexcitability that is associated with epilepsy. Taken together, this proposal aims to understand circuit rearrangements functionally affect the epileptic hippocampus, so that future treatments can target the disease of epilepsy, rather than the symptoms.